1. Field of the Invention
The present invention relates to raise-type earth boring drills, and more particularly to such raise drills in which the drive stem is removably attached to the raise bit body.
2. Description of the Prior Art
A relatively large diameter hole may be provided between two locations in a mine by an operation commonly referred to as raise drilling. A raise drilling operation begins by drilling a small diameter pilot hole through the earth between the locations using a small diameter pilot bit. After the pilot hole is completed, the pilot bit is removed from the drill column and a large diameter raise bit is attached. The raise bit is then rotated and drawn along the pilot hole to enable the drill cutters to contact and disintegrate the earth formations surrounding the pilot hole, thereby enlarging the pilot hole to the desired size. In an exemplary embodiment, the pilot hole may be 11 inches in diameter and the reamed out hole may be 6 feet in diameter.
During a raise drilling operation, a tremendous amount of wear and stress is imposed upon the raise bit. The drive stem in particular is subjected to considerable wear due to abrasive contact with the surrounding earth formation and is also subject to considerable stress resulting from (a) tension due to the pulling force imparted to the drill, (b) twisting due to the torque applied to the drill, and (c) bending due to uneven loading around the circumference of the drill.
The advantages of having the drive stem removable are that the elements having a relatively short lifespan can be replaced, thereby extending the useful life of the bit, and the low profile of the separated components allows the raise bit to be transported through small drifts or passages.
Various problems have been encountered in utilizing prior art raise bits having removable stems. Quite often, special torquing tools are required to disassemble and remove the drive stem from the raise bit body. Such tools are not always readily available in the mining tunnels where the equipment is being used.
In many prior art devices, the torsional forces are transmitted through conical or circular collars which is less positive than desired.
Many prior art devices also do not provide for proper alignment of the stem attachment. As a result serious bending stresses can be encountered due to misalignment of the drive stem with the raise bit body.